1. Field of the Invention
This invention relates generally to turbine blade root design and, more particularly, to the mounting of turbine blades in side-entry grooves and methods for forming the grooves.
2. Description of the Related Art
A steam turbine can include a combination of low pressure, intermediate pressure, and/or high pressure steam turbine elements which are coupled together to provide a single power output. Each steam turbine includes a rotor having a plurality of rotating blades mounted thereon in grooves. Usually, the blades of a given row are identical to each other. The rotating blades of a row extend radially outwardly from an outer surface of the rotor, with the rows being spaced apart. The rotating blades of one row differ in shape from those of the other rows; most noticeably the rotating blades of each row, or stage, vary in length depending on position along the rotor.
Each rotating blade, regardless of row, has a foil portion extending radially outwardly from the rotor and a base portion for mounting the blade to the rotor. The base portion includes a root which is fitted into a mounting groove provided for each blade of a row, and can include a platform integrally formed at the proximal end of the foil portion. The foil portion has a tip at the distal end and may have a twist profile from the proximal end to the distal end, or may be parallel-sided. Sometimes, shrouds are provided at the tips as separately added or integrally formed components.
A stationary cylinder is coaxially supported around the rotor and has a plurality of stationary blades mounted on an inner surface thereof. The stationary blades are arranged in rows which, when the cylinder is assembled with rotor, alternate with rows of rotating blades. The stationary blades of one row are shaped differently from those of the other rows, although all stationary blades have a foil portion. Some stationary blades have a base portion which includes a root and a platform. Others have the foil portion welded directly into the blade rings with no root or platform.
Rotor blade grooves provided in the rotor for mounting the rotor blades are usually geometrically more complex than the mounting grooves provided for stationary blades. Moreover, the roots of the rotating blades and the rotor are subjected to substantially greater stresses than corresponding roots of stationary blades.
Some turbines have turbine rotor blades mounted in what are referred to as "side-entry" grooves provided in the rotor. When mounted, the rotor blades extend radially outwardly from the rotor in rows which are disposed circumferentially around the rotor. Instead of having a single annular groove for mounting the plurality of rotor blades which constitute a row, a side-entry groove arrangement includes, for a given row, a series of spaced apart side-entry grooves, each side-entry groove of the series being provided for each rotor blade of the row.
A typical side-entry groove starts at the outer surface of the rotor as an opening which tapers inwardly towards a bottom of the groove. A series of undulations are provided between the opening and the bottom of the groove symmetrically on opposite sidewalls of the groove. A typical root of a corresponding turbine blade has a shape which substantially conforms to that of the groove. The undulations provide a series of interlocking steps.
The resulting shape of the rotor grooves and blade root is sometimes referred to as a fir tree.
In a side-entry groove, the root is pushed into the groove along a path lying in a plane perpendicular to the turbine rotor radial direction, and therefore, an interlocking can be achieved. Tolerances for both root and groove are very precise. Root and groove contour tolerance envelopes typically allow variations of 0.006 inches (0.15 mm) along the non-contact surfaces, with much smaller variations permitted on the contact surfaces. Basically, a precise fitting between the root and the groove is required such that the maximum clearance between the root and the groove is extremely small.
There is a general reluctance to change rotor blade root and groove configurations once a particular design has been developed. This is because it may have taken months or even years of meticulous calculation to arrive at a particular design. Sometimes, slight variations in rotor blade root and groove profiles lead to unacceptable decreases in the function or performance of the blades or the rotor. Given that the tolerances between the root and the groove are critical, changes in the profile of either or both goes against conventional wisdom.
Ordinarily, the root of a side-entry rotor blade fits into the groove which has a shape nearly identical to that of the root. This is done in order to minimize losses associated with leakage of the motive fluid. An exception to this practice sometimes occurs in high-temperature applications, where clearances are introduced between the bottom of the root and the bottom of the groove to provide a passage through which a cooling medium can pass.
Fir-tree blade roots and their corresponding mounting grooves are widest at their locations nearest to the foil and narrowest at their locations nearest the rotor body. This is done in order to most efficiently exploit the material which is available to transmit loads from the blade to the rotor, and to provide for generous fillet radii which serve to minimize stress concentration effects.
Because the sides of the blade root are unobstructed during manufacture, the cutting devices (machine tools, grinding wheels, or broaches) which are used to make the root can be constructed to be arbitrarily massive and, stiff. Groove cutting, however, is relatively much more difficult. One problem associated with groove cutting is that the size of the cutting tool is necessarily restricted to the size of the groove which is being cut.
If the bottom neck of the groove is not sufficiently large, then the bottom-most portion of the groove cutter will be weak and flexible. Among the possible undesirable consequences are the following:
(1) the groove cutter may break off during the cutting operation, potentially rendering useless the rotor which is being machined; (2) flexing of the cutter will remove extra material from the bottom contact surfaces of the groove. When a blade is assembled into such a groove, the bottom lug will not carry its intended portion of the total load. The remaining lugs will then be forced to carry more than their intended load, with adverse effects on reliability and life of the blade attachment structure.
To avoid these undesirable consequences, it is frequently necessary to compromise the strength of the blade fastening design by making the bottom neck of the groove wider than would otherwise be ideal.